Method for preventing snaking of continuously cast metal slab

ABSTRACT

Molten metal is poured into a continuous casting mold having a rectangular mold exit opening to produce a rectangular metal strand having a liquid core, a width and a thickness. A convex shape is imparted to the rectangular strand along at least a part of the width thereof downstream of the casting mold by arranging a plurality of oppositely disposed guide rolls immediately downstream of the rectangular mold exit opening so that the cast strand while travelling from the mold exit opening through the plurality of guide rolls expands along at least part of its width no more than 8% of the strand thickness, thereby preventing snaking of the strand.

BACKGROUND OF THE INVENTION

The invention is directed to a strand guide stand which prevents aweaving motion of a cast strand.

DESCRIPTION OF THE PRIOR ART

Strand guide stands in which the rolls have curved rather than straightsurface lines are known from the prior art, e.g., DE 36 27 991 C2. Suchrolls are used whenever the mold outlet cross section produces a strandcross section substantially deviating from the rectangular form.

Generally, for rectangular strands with strand thicknesses greater than60 to 80 mm, rolls with straight surface lines are used in the strandguide stand directly below the continuous casting mold (segment 0) andthe surface lines of rolls located opposite one another in pairs lieparallel to one another.

In strand guide stands having such rolls, the casting speed is limitedby a weaving or wobbling motion of the strands so that the followingvalues are conventionally applied:

approximately 1.8-2.0 m/min for slabs with a thickness of 230 mm

approximately 1.5-1.7 m/min for blooms with a thickness of 270 mm

approximately 2.5 m/min for billets with a size of 100×100 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic view of the rectangular mold and cast strand ofthe instant invention;

FIG. 1b is a schematic side view of FIG. 1a;

FIG. 2 is a partial cross-sectional view of the cast strand permitted toexpand due to the concave surface line of the guide rolls;

FIG. 3a is a schematic view of the guide rolls of the instant inventionin parallel and position; and

FIG. 3b is a view of the guide rolls in concave position.

SUMMARY AND DESCRIPTION OF THE INVENTION

The object of the present invention is to provide rolls for the strandguide stands for rectangular strands which rolls are designed so that aweaving or wobbling motion of the strands is prevented.

Pursuant to this object, and others which will become apparenthereafter, one aspect of the present invention resides in a strand guidestand for a continuously cast rectangular strand, which guide standincludes guide rolls and driving rolls located opposite one another inpairs. The rolls have a slightly concave surface line, at least whenloaded, so that the strand is accurately guided without weaving. Thisconfiguration of the rolls permits the strands to expand sphericallyupon exiting the continuous casting mold.

In another embodiment of the invention the rolls are configured to havea concave surface line with a maximum concavity of no more than 8%, andpreferably 2-4%, of the strand thickness.

In still another embodiment of the invention the strand guide stand isconfigured to bend under casting load in a range of elasticity so as toform the concave surface line of the guide rolls and the driving rolls.

In yet another embodiment of the invention, the rolls are configured sothat the concave surface line extends over a part, preferably 40%, ofthe strand width.

It has been shown that when using continuous casting molds with arectangular cross section and cylindrical guide rolls, which ispresently the case in the majority of continuous casting installationsthroughout the world, the casting speed may not exceed specified maximumvalues because, otherwise, the number of breakout defects in the strandincreases disproportionately. This is a result of the fact that thestrand begins to weave, that is, to move back and forth between theguide rolls, if a determined casting speed is exceeded. This leads to anuneven transfer of heat at the strand in the vicinity of the mold outletcross section, which results in stress cracks and breakout.

Surprisingly, it has now been found that a slight camber of the strandprevents weaving. In so doing, the strand expands to the extentdetermined by the concave surface lines of the rolls immediately uponexiting the mold due to the still liquid core and the small thickness ofthe strand shell. Thus, the strand contacts the rolls and is guided soas to prevent weaving, or "snaking" as it is called in the technicalliterature.

The camber can extend over a part of the strand width or over the entirewidth of the strand. The total bulging of the strand should amount to8%, preferably 2-4%. Increasing this value does not improve guidance,but only results in unnecessary rolling effort. A camber along theentire width of the strand in the range of 2-4% provides a substantiallybetter initial cross section for the subsequent rolls and meets therequirements of rolling mill engineers in this respect. The camber ofthe strand is expediently provided by means of (for example) concaverolls arranged up to the end of the strand guide stand or by means of acorresponding arrangement of split rolls in the strand guide area or bymeans of bending of the strand guide stands and/or of the rolls in theelasticity range.

The present invention not only makes possible a corresponding increasein the casting speed in new installations, but also allows the castingspeed in existing installations to be increased by up to 200% comparedwith the values indicated above by retrofitting with suitably camberedrolls pursuant to the present invention while at the same timeincreasing the reliability of the casting process.

What is claimed is:
 1. A method of preventing snaking of a strand ofcast metal in a continuous casting device, comprising the steps of:(a)pouring molten metal into a continuous casting mold having a rectangularmold exit opening to produce a rectangular metal strand having a liquidcore, a width and a thickness; and (b) imparting to said rectangularstrand a convex shape along at least a part of the width thereofdownstream of said casting mold by arranging a plurality of oppositelydisposed guide rolls immediately downstream of said rectangular moldexit opening so that the cast strand while travelling from said moldexit opening through said plurality of guide rolls expands along atleast a part of its width no more than 8% of said strand thickness,thereby preventing snaking of the strand.
 2. The method of claim 1,wherein the step (b) is performed by permitting the cast strand toexpand no more than 2-4% of the strand thickness.
 3. The method of claim1, wherein the convex shaped is imparted to the rectangular strand bybending the guide rolls in the range of elasticity under cast load. 4.The method of claim 1, wherein the convexity of the strand extends alongthe entire width thereof.
 5. The method of claim 1, wherein theconvexity of the strand extends along 40% of the strand width.